FIG. 1A shows an exemplary optical layout of a test set configuration, generally designated as 10, for testing a primary mirror (PM), for example. The primary mirror 12 is shown as a monolith structure, but may be constructed from multiple mirrors segments that typically require alignment before general use. A test system 11 transmits rays of light 17 toward PM 12. The rays of light 17 are reflected off PM 12 and returned to test system 11 for aligning the mirror segments of PM 12.
The test system 11 may be seen in greater detail in FIG. 1B. As shown, test system 11 includes interferometer optics 13 and nulling device 14. The rays of light 17 are emitted from the optics of the interferometer toward nulling device 14. The light rays first enter spherical imaging mirror 15 through a first aperture (not shown) and then reflect off aspheric mirror 16 back to the spherical imaging mirror. The light rays 17 exit nulling device 14 through a second aperture (not shown) in aspheric mirror 16. FIG. 1B also shows the PM paraxial focus, which is referred to as a center of curvature (CoC) 18 of the PM.
FIG. 1C shows the test system from front focus 9 of the objective lens to interferometer sensor 3. Also shown is back focus 7 of the objective lens, which is the entrance port of the interferometer. It will be appreciated that the image is formed at the interferometer sensor, and the PM is treated as the object. The aperture of the internal relay in the interferometer is shown as internal stop 5 of the system and is also the image conjugate to the front focus of the objective lens, where the image of the internal stop has a diameter of approximately 1.5 mm.
It will be understood that the imaging of the PM, shown in FIGS. 1A-1C, is a single pass. In other words, each surface in the ray trace is impinged only once. When performing interferometry all surfaces, except the PM, are impinged twice. This is because the interferometer transmits the light source toward the PM, and receives the same light reflected back from the PM. In this example, images of the PM may be formed at aspheric mirror 16, objective back focus 7 and sensor 3. These surfaces are, thus, defined as conjugate to the PM, or the pupil conjugate.
Optical surfaces may be calibrated using a computer generated hologram (CGH). For example, the PM surface shown in FIG. 1A may be tested or calibrated using a CGH. Furthermore, the optics in the test system, for example, optics in the interferometer and/or optics in the nulling device may be calibrated using a CGH.
Referring to FIG. 2, ray traces are shown between the objective lens front focus (part of interferometer optics 13 shown in FIG. 1B) and the center of curvature (CoC) 18 of PM 12. The CGH 19 may be placed at or near the CoC. The CGH is, typically, moved into position to intercept the light arriving from the interferometer during calibration; and, typically, is moved out of position, when calibration is completed.
The CGH acts as an inverse null lens. The CGH provides light back to the objective lens' front focus 9 without any wavefront variance, if everything is perfect (at 687 nm wavelength, for example). The CoC is usually located outside the caustic to provide easier wavefront mapping. (The caustic ends at the CoC.) The CGH may be located anywhere between the CoC and the PM, but is typically located close to the CoC to keep the radial size of the CGH to a minimum.
Thus, during an exemplary calibration performed by the inventor, the PM was replaced with the CGH. The CGH was located between the CoC and the PM (5 mm away from the CoC). The image was analyzed at the interferometer entrance port. As a result of the CGH located 5 mm from the CoC, the CGH was not a pupil conjugate of the interferometer pupil. The interferometer pupil, therefore, was not imaged well back on itself.
Results of the calibration is shown in FIG. 3, which provides a plot of the root mean square (RMS) wavefront error (WFE) in waves, at 687 nm, versus a Y-field in mm for the pupil image. The image was taken with a ray trace from the first pass aspheric mirror of the nulling device to the interferometer entrance port. The large WFE is due to a large field curvature at or near the axis. The WFE is lowest, however, at the edge of the pupil image field. Interestingly, the WFE also increases as the CGH location is moved closer to the CoC.
The WFE shown in FIG. 3 is due to a finite conjugate ray trace from the first pass aspheric mirror 16 to the interferometer entrance port 7. Here the aspheric mirror 16 is considered the object to be imaged back to the interferometer entrance port 7, or back focus 7.
This contrasts sharply with the imaging results of the test configuration shown in FIG. 1A, in a single pass ray trace having an axial length of 3300 mm between the interferometer entrance port and the PM under test. The WFE for the test configuration of FIG. 1A is shown in FIG. 4. As shown, the WFE results are well within the diffraction limit of 0.07 RMS WFE. Accordingly, there is a wide discrepancy in imaging test results between the test configuration of FIG. 1A and the calibration configuration of FIG. 2. This discrepancy must be minimized for good calibration fidelity.
The present invention provides a system and method for improving test results, when using a CGH to calibrate a wavefront measuring system (WMS), such as an interferometer and a reflective nulling device. The present invention is described below.